During a normal life cycle, two types of blood vessels exist: the resting and quiescent fully developed blood vessels in adults, and the proliferating or developing new blood vessels that are found only in early development and reproduction, e.g., menstrual cycle and pregnancy. In contrast, angiogenesis, the proliferation and development of new blood vessels, occurs physiologically in wound healing and pathologically it is associated with processes like tumor growth and macular degeneration. Angiogenesis is a complex process involving many stages, including extracellular matrix remodeling, endothelial cell migration and proliferation, capillary differentiation, and anastomosis.
Angiogenesis can be a target for treating diseases characterized by abnormal neovascularization. The presence of extra blood vessels where there should be none affects the mechanical properties of a tissue, increasing the likelihood of failure. This is illustrated by wet macular degeneration in which the local expansion of blood vessels threatens macular vision. In addition, all detectable solid tumors (tumors over 2 mm in diameter) need angiogenesis to deliver oxygen and nutrients to actively proliferating tumor cells and remove cellular wastes. The level of vascularization in a tumor is also associated with metastasis of cancer cells.